Steel and method of treating the same



Patented Se t. 4,1934

UNITED STATES PATENT OFFICE 1,972,241 v STEEL AND METHOD or TREATING THE SAME ration of New York L. No Drawing. Application May 3, 1932,

Serial No. 609,024

4 Claims.

This invention relates to steel and method of treating the same. It has to do, particularly, with a method of making steel rails, although it is not, necessarily, limited thereto.

In the prior art, numerous efforts have been made to increase the tensile strength, elastic limit, toughness, hardness, ductility, et cetera, of steels. These efforts have met with some degree of success but limitations have been encountered in the quality of the material produced and in the cost of producing this material.

For example, in the making of steel rails, where the increased weight, speed and volume of rail!- way trafiic have greatly raised the requirements along the lines indicated, efforts have been made to meet these requirements by the introduction of additional carbon in the steel. This, however, has led to undesirable brittleness. In an effort to reduce this brittleness, the additional steps of quenching and tempering have been resorted to experimentally, but this, necessarily, involves expensive equipment and other difiiculties in production, such as added cost, reduced output, et cetera.-

Other efforts to. obtain increased tensile strength, elastic limit, toughness, hardness, ductility,- et cetera, have involved the use of copper and other alloying elements. Such other alloying elements are comparatively expensive, are sometimes oxidized and lost in remelting and tend to enter the slag with resultant disturbances of the steel-making process.

One of the objects of this invention is to provide steel rails having a high elastic limit, high tensile strength, adequate ductility, suitable hardness and resistance 'to impact and adequate endurance limit.

Another object of this invention is to provide a steel rail having the characteristics indicated above as desirable, without resorting to the costly steps of quenching and tempering and without materially departing from the normal methods of present-day manufacture.

Another object of our invention is so to control the manufacture of steel rails that the danger of the occurrence of transverse fissures will, at least, be materially decreased.

Another object of our invention is to provide a method of improving the physical properties steel an adequate quantity of copper and subsequent subjection of the copper and steel rails to a precipitation-hardening process. It involves the cooling of the steel rail with suflicient rapidity to maintain the copper in solution in the ferrite coupled with the subjection of the copper steel rail to a temperature and for a period of time, with both the temperature and the period of time being selected and definitely controlled, so as to bring about a precipitation-hardening action. Furthermore, by the control of this temperature and of the time period, we are able to so control the precipitation-hardening action that the degree of hardening, the tensile strength, the elastic limit, the ductility, et cetera, may be regulated so as to produce a copper steel having these qualities in selected degrees.

This method may be utilized to advantage with steels of any desirable carbon content, and whether the steel is cast, rolled or forged to shape.

More specifically, the method which we utilize is best illustrated by the following series of steps which we have developed during the course of our investigations:

The first step includes the addition of copper to the steel during some convenient part of the steel-making process, either before or after melting. This may be done without otherwise changing ordinary practice, for the copper is not oxidized or lost in remelting, and it does not enter the slag to disturb the steel-makin process as is now the case with many other alloying elements.

.The percentage of copper which we may add to the steel may vary within wide limits, such as 0.50% to 5.00%. In general, we find the optimum percentage to lie between 0.60% and 2.00 with some adjustments within or somewhat beyond that range, depending upon the carbon content of the steel and the presence or absence of other alloying elements.

For rails of carbon steel with normal manganese content, we ordinarily prefer to hold the carbon within the range 0.50% to 0.80% carbon, and for medium manganese steel toward the middle or lower end of that range. For objects other than rails, still higher carbon may be used.

We find that in strengthening a steel by the use of copper and the reheating process the best combination of strength and toughness will usually be obtained if the carbon content of the carbon or alloy steel is slightly reduced from that which would give the best results in the absence of copper.

Following the addition of the desired amount of copper, the ingot is formed and the rails formed by the usual rolling or forging operations, no departure from ordinary good practice in this respect being required.

After the rolling or forming operation is completed, the rails are cooled or permitted to cool at a rate fast enough to retain the bulk of the copper in a solid solution, so that precipitationhardening may be produced by reheating. Under the method which we use for reheating, it is possible to produce precipitation-hardening, even though the metal has been initially cooled at as slow a rate as 24 C. per hour, though cooling at a rate less than 180 per hour tends to cause a decrease in the precipitation.

Furthermore, it is desirable that the initial cooling from the rolling temperature through the range of secondary brittleness be retarded to such an extent as to avoid such marked differences in temperature in various parts of the rail as would produce or tend to produce shatter cracks.

If the cooling range is inadvertently made too slow, the material may be regenerated by again heating to a temperature above which the solubility of copper in the copper-ferrite solid solution is a maximum, for a suflicient time to bring the copper into solution, and then cooling at the proper rate.

After the rail is formed and properly cooled with the greater part of the copper remaining in solid solution, we subject the rail to a controlled reheating operation to bring about the effect known as precipitation-hardening.

This operation consists of reheating the rail to a selected temperature which may be varied between about 350 C. and 625 C., holding the rail at the selected temperature for a definite length of time, and then permitting the rail to cool.

We have found that for each composition of steel there is an optimum temperature to which the rail should be heated, and an optimum length of time that it should be held-at the reheating temperature, in order that the maximum hardness values may be attained, and beyond which the beneficial effects of the reheating are lost.

As stated before, the temperature and the time involved in the hardening process vary with the composition of the steel. They may be determined readily by simple routine tests. Also, by suitable change in either the time or the tem perature, or both, of the reheating process, the physical characteristics of the finished rail may be varied through a wide range at will without the necessity of altering the composition of the steel or other changes in the rail-making process.

We have provided a means whereby steels .having a wide range of physical properties, adapted to a variety of purposes may be produced at will from a cheaply and easily prepared alloy by a simple, controlled reheating process, without the nleicessity of changing the chemical nature of the a oy.

As examples of our invention, we cite the following tests which have been made by us:

Steel of 0.56% C, 0.60% Mn, 0.21% Si, 041% P 0.029% S, called heat 11, and one of 0.74% C, 0.45% 'Mn, 0.17% Si, 0.035% P, 0.026% S, called heat 3-1, (both containing but 0.07% Cu carried in from the scrap charge) divided into three parts and to two of these about 1.00% and 1.50%

Cu, respectively, were added, and the resulting steels cast separately for comparative tests. The analyses of steels of various copper content and the tensile strength of forged bars, as normalized from 850 C. were:

Tensile Yield Elong. Red. 01' Heat 0 Mn strength strength in 2" area Upon precipitation hardening by reheating heat 1 to 450 C. for four hours, and heat 3 to 500 C. for three hours, these steels showed:

Tensile Yield Elong. Bed. of Heat Cu strength strength in 2" area l-l 0. 07 114, 500 64. 500 17 32 .55.56 C l-2 1. 13 136, 500 89, 500 13% 23 l3 l. 64 143, 500 90, 000 12 23 3-1 0. 07 120, 000 56, 000 19 25% .71.74 C- 3-2 1.07 149, 500 07, 250 14 25% 3-3 1. 47 149, 000 102, 000 my, 26%

Furthermore, we have found that the Brinell hardness of the forged steel materially increases after a precipitation hardening treatment. As an example, steels 11 and 31 tested as follows following reheating at 450 0.:

Brinell hardness after reheating for Heat Cu 0 hrs. 2 hrs. 4 hrs.

1-1 0. 07 203 209 217 .55.56 C l-2 1.13 223 231 280 l-3 1. 64 255 205 285 The following test of a steel with 0.55% carbon and 1.13%copper normalized and then reheated for various periods at a series of reheating temperatures will serve to illustrate and emphasize the importance of the relationship between the temperature and the duration of the reheating process and the resulting hardness of the steel:

Precipitation hardening progresses very rapidly at higher temperatures, but the magnitude of the effect diminishes until at temperatures somewhat above 625 C. it disappears entirely.

The magnitude of the hardening effect at any particular temperature is but slightly affected by the composition of the steel unless the copper content is lower than 0.50% or greater than 2.00%. Beyond these copper content limitations the precipitation hardening effects disappear rapidly.

From the tests above, it is apparent that from the original carbon steel we have, in two simple steps, comprising the addition of 1.64% of copper and a reheating operation, been able to increase the ultimate strength of the steel 33,500 pounds, the elastic limit 30,500 pounds, the Brinell hardness 62 points, and the elastic ratio 12.60%.

It is also evident that, in addition to greatly improving the physical properties of the steel, we have also imparted to it the substantial resistance to corrosion due to the presence of copper.

We have also found that the same controlled hardening effect may be obtained without the necessity of reheating, by cooling the rail at the conclusion of the rolling operation from rolling temperature down to the selected precipitation temperature, holding it at the selected temperature for the selected length of time, and' then cooling to room temperature in the usual way. In this way it is possible, when convenient to do so, to combine the rolling and hardening operations and efiect a further saving in time and in the fuel necessary for reheating.

Since precipitation hardening of the copper steel results from placing the copper in solution with the ferrite and then cooling at the rates indicated, it will be apparent that we may perform this operation in any one of several ways. Thus, we may cool the melt directly down to the requisite stage at a sufficiently rapid rate to maintain the copper in solution with the ferrite. 0n the other hand, as indicated, we may permit the copper steel to cool to room temperature and then reheat it to a temperature at or above 730 C., at which temperature maximum solubility of the copper in the ferrite is obtained.

It will also be apparent that some of the commonly used heat treating methods may be relied upon to insure the attainment of a temperature which will bring the copper into solution in the ferrite. For example, as indicated, the normalizing treatment which ordinarily involves heating the metal to a temperature in excess of 750 C. may be relied upon to bring the copper into solution in the ferrite and, of course, the annealing treatment commonly used will serve the same purpose. Likewise, as indicated, the hot rolling treatment such as is commonly used in producing railroad rails will insure a temperature more than adequate to place the copper in solution with the ferrite, while maintainingthe grain size of the metal sufficiently small to insure that the precipitation hardening treatment may be made effective. Obviously, such treatments as forging or the like may be made to serve the same purpose.

In view of these facts, it is within the scope of our invention to cool directly from the molten state to produce solid solution of the copper in the ferrite, to reheat for the special purpose of attaining the desired .solubility or to attain this desired solubility by heat treatments of the nature indicated.

Thus, it will be seen that we have provided a novel composition of steel and a novel meth 0d of treating the same which enables us to increase the tensile strength, elastic limit, tough- .ness, hardness, ductility, et cetera, thereof without resorting to the steps of quenching and tempering with their consequent drawbacks of expensive equipment, reduced output and other undesirable features. It will, furthermore, be apparent that our method enables us to use steels with higher carbon content than have previously been considered available without quenching and tempering. It will also be apparent, however, that our method is applicable to steel of low carbon content as well as steel of high carbon content and that it renders possible the production of steel possessing wide ranges of desirable ,properties from an alloy that may be produced readily and at a low cost.

Having thus described our invention, what we claim is;

1. The method of making steel which comprises alloying therewith from 0.50% to 5.00% copper, cooling the steel at a rate of 24 C. to 180 C. per hour to maintain a substantial quantity. of the copper in solution and until a temperature between 625 and 350 C. is reached, and maintaining the steel at a selected temperature within said temperature range for a period of time ranging from 15 minutes to 6 hours, the interval of time within said time period being dependentupon said selected temperature, to thereby eifect precipitation hardening of the copper steel.

2. The method of making steel which comprises alloying therewith from 0.50% to 5.00% copper, cooling the steel at a rate of approximately 180. C. per hour to maintain a substantial quantity of the copper in solution and until a temperature between 625 and 350 C. is reached, and maintaining the steel at a selected temperature within said temperature range for a period of time ranging from 15 minutes to 6 hours, the interval of time within said time period being dependent uponsaid selected temperature, to thereby effect precipitation hardening of the copper steel.

3. The method of making steel containing up to 1.00% carbon and up to approximately 2.00% manganese which comprises alloying therewith from 0.50% to 5.00% copper, cooling the steel at a rate such as to maintain a major portion of the copper in solution and until a temperature between 625 and 350 C. is reached, and maintaining the steel at a selected temperature within said temperature range for a period of time ranging from 15 minutes to 6 hours, the time interval within said time period being dependent upon said selected temperature, to thereby effect precipitation hardening of the copper steel.

4. The method of making steel containing up to 1.00% carbon and up to approximately 2.00% manganese which comprises alloying therewith from 0.50% to 5.00% copper, heating the steel to a temperature and for a sufiicient time to bring the copper into solution, cooling the steel at a rate such as to maintain the copper in solution and until a temperature between 625 and 350 C. is reached, and maintaining the steel'at a selected temperature within said temperature range for a period of time ranging from 15 min- 

